With the development of sciences and technologies, more and more electronic systems are required to be applied in various radiation environments, for example, space radiation environment, nuclear radiation environment, simulated source environment and ground radiation environment. Various radiation effects may make an integrated circuit fail, and single event effect has become a main threat that causes a thin gate oxide device to fail. The single event effect refers to that when a highly energetic charged particle in a radiation environment passes a sensitive region of an integrated-circuit device, a large number of electron-hole pairs generated on the trace are collected under a force of an electric field of a semiconductor PN junction depletion layer, which causes a logic state of the device to change abnormally or causes the device to be damaged. Wherein, the charges collected may cause a transient current at a circuit node, and the phenomenon is referred to as single event transient. If the single event transient occurs in an output stage of a circuit, an error in the output may be caused; if the single event transient occurs in a storage cell, information stored in the storage cell may be caused to change; moreover, if occurs in a logic circuit, the single event transient may propagate along the logic circuit, and a propagation induced pulse broadening (PIPB) effect may appear during the propagation process, which aggravates a probability that the single particle causes an integrated circuit to fail. Since, as the integrated-circuit device works normally, a drain junction is usually reversely biased, the drain junction of the device is regarded as a sensitive node on which charges are collected due to the single event effect. Therefore, it is important to the research of radiation-hardened circuits that the number of charges collected on the sensitive node of the integrated-circuit device in a radiation environment is lowered.
FIG. 1 is a schematic diagram showing a process in which charged heavy ions pass through a sensitive node of a device. In the figure, reference sign “a” refers to a depletion region, reference sign “b” refers to a “funnel” region, reference sign “c” refers to electron-hole pairs generated along an ion trace, and reference sign “d” refers to the ion trace.
A basic process for forming single event transient in a radiation environment includes the following stages.
1) Charge Generation: Charged ions are injected into a semiconductor material, and because the ions have a very high energy, a large number of electron-hole pairs are generated along an ion trace.
2) Instantaneous Collection (also referred as Funnel Collection): the large number of electron-hole pairs generated make a depletion region of an original PN junction to locally collapse, and make the iso-electrostatic potential surface of the depletion region deformed to form a shape similar to a funnel. The electron-hole pairs in the funnel start to drift in different directions and are collected by the electrodes under the force of an electric field of the depletion region. The duration of such a process is usually tens of ps to hundreds of ps, and is called instantaneous collection.
3) Diffusion Collection: the electron-hole pairs outside the depletion region diffuse due to a concentration difference. If the electron-hole pairs diffuse into the PN junction which is reversely biased, the electron-hole pairs are again collected by the electrode under the force of the electric field of the depletion region. Because there is no force of an electric field, most of the electron-hole pairs are recombined, and the charges diffused or collected do not occupy a dominant position among the charges collected in the single event transient.
Since, during a normal working process of a planar CMOS device, the drain region and the substrate underneath are usually in a reversely biased state, the drain region of the device is usually a sensitive node in a radiation environment. Since the substrate is lightly doped relative to the drain region, a dimension of the funnel region generated in single event transient is very large, and a dimension of the funnel region is a key factor affecting the number of charges collected by the sensitive node.